In electronics, filler-containing moulding compounds based on epoxy resins are used for the encapsulation of active and passive components. Examples of such applications are the encapsulation of capacitors, diodes, transistors, power semiconductors, optocouplers, memory chips and microprocessors. Suitable epoxy-resin moulding, compounds for this purpose must fulfil high requirements relating to the processing and moulded-material properties. This relates, in particular, to the purity of the moulding compounds and also to the rheological behaviour and the curing properties during processing as transfer moulding compounds by means of transfer moulding processes and, furthermore, to the mechano-thermal properties of the epoxy-resin moulded materials and to the stable long-term protection of the components against aggressive environmental influences. In addition, the epoxy-resin moulded materials must fulfil the high requirements of electronics relating to the flame-resistant behaviour and achieve the classification V-0 for a layer thickness of .ltoreq.1.6 mm in the internationally standard flammability test according to UL 94 V.
The curing of epoxy-resin moulding compounds can in principle be carried out with chemically varied hardener components, for example with carboxylic anhydrides, amines or phenols. However, phenolically curable epoxy-resin transfer moulding compounds heavily filled with synthetic silica flour have gained acceptance for the encapsulation of electronic components by means of transfer moulding processes.
The phenolically curable epoxy-resin transfer moulding compounds at present used industrially contain, as a rule, 10 to 30% of organic components and 70 to 90% of inorganic components. In most cases, the chemical basis of the resin components is composed of cresol-novolak epoxy resins. Phenol novolaks are predominantly used to cure the epoxy resins; the reaction is accelerated, for example, with triphenylphosphine, phenylphosphonium borate and 2-ethyl-4-methylimidazole. Silicone-modified epoxy-resin components are used in the moulding compounds to improve the low-stress behaviour (in this connection, see German Offenlegungsschrift 42 23 632).
The flame-resistant formulation of the compression-moulding compounds is carried out using aromatic bromine compounds, in particular with tetra-bromobisphenol A epoxy resins, which are chemically incorporated in the epoxy-resin moulded material during curing. In addition, antimony trioxide increases the effectiveness of the brominated flame retardants as a synergist. The flame-resistant properties of the epoxy-resin moulded materials are further promoted by a high content of silanized synthetic silica fillers; in this connection, both splintery fillers and mixtures of splintery and spherical fillers are used. Recently, synthetically produced .alpha.-radiation-free synthetic silica fillers have been used to avoid so-called soft errors in large-scale-integrated circuits. The high content of silanized synthetic silica filler also serves to improve the mechano-thermal properties of the epoxy-resin moulded materials, in particular to adjust the coefficients of expansion. The compression-moulding compounds also contain, in addition, small amounts of additives, in particular soot, and processing aids, such as stearates and waxes.
To produce the phenolically curable epoxy-resin transfer moulding compounds, the resin and hardener components are converted into a pre-reacted state (B stage) and pulverized, as a rule, at temperatures up to approximately 120.degree. C., in particular by kneading on a roll mill or by extrusion, for example by means of a screw compounder. To process the transfer moulding compounds by means of transfer moulding processes, the moulding-compound powder is generally pelletized and, if necessary, the pellets are pre-heated to 70 to 90.degree. C. before processing. The components are generally encapsulated at moulding temperatures from 170 to 190.degree. C. and a pressure of 70 to 200 bars, the compression-moulding compound being cured in the mould generally for 60 to 120 s. The components are then demoulded and, if necessary, postcured at temperatures from 170 to 190.degree. C.
Although the phenolically curable epoxy-resin moulding compounds which have been flame-resistantly formulated with bromine-containing flame retardants and antimony trioxide and which have outstanding flame-retarding properties have proved advantageous in electronics, their use is not, however, without problems. In particular, during fire and carbonization, they form highly corrosive bromine compounds, such as hydrogen bromide, and aromatic bromine compounds which are biologically difficult to degrade, such as polybromodibenzodioxins and polybromodibenzofurans. In addition, epoxy-resin moulded materials containing brominated flame retardants are unsuitable for recycling if a further scattering of dangerous products is to be avoided. In addition, the disposal of such epoxy-resin moulded materials as waste by combustion can be carried out in future only as hazardous waste by means of industrially expensive and economically unprofitable combustion processes because of the ever-increasing requirements relating to air-pollution prevention. One objection to the use of antimony trioxide is that this compound is on the list of carcinogenic hazardous substances. Although the risk is negligible in the case of permanent incorporation, as exists in the cured compression-moulding compound, expensive protective measures relating to health and safety at work are however necessary for the production of antimony-trioxide-containing compression-moulding compounds. In addition, antimony trioxide which is released during the processing, i.e. during the encapsulation of electronic components, and also in the event of fire or carbonization or during recycling processes represents a high risk in the form of inhalable dust.
Work has also already been carried out on the flame-resistant formulation of compression-moulding compounds without the use of halogen-containing flame retardants. For this purpose, German Offenlegungsschrift 42 23 632 discloses a latently reactive, phenolically curable epoxy-resin moulding compound without halogen-containing flame retardation, in whose production the following procedure is adopted. First of all, an isocyanate-group-free, latently reactive prepolymeric epoxy-resin mixture is produced in powdered form from a filler-containing, thermally polymerizable reaction resin mixture of polyepoxy resin EP (mixture of difunctional and multifunctional epoxy resins) and polyisocyanate resin IC (EP:IC molar ratio &gt;1) using substituted imidazole as a reaction accelerator (concentration: 0.5 to 2.5%, relative to EP) at reaction temperatures up to 200.degree. C. Said epoxy-resin mixture is then mixed with a powdered, filler-containing phenolic-resin mixture (molar ratio of epoxy groups to phenolic hydroxyl groups: 1:0.4 to 1:1.1), optionally with the addition of additives.
A disadvantage of this process is that, in the production of the prepolymeric epoxy-resin mixture, which is carried out, for example, by means of a continuously running reactor (in this connection, see German Offenlegungsschrift 42 23 622), high contents of filler are necessary in order to "dilute" the reaction resin mixture, i.e. to reduce its reactivity and to be able to remove the heat of reaction released during the reaction of the polyepoxy resin with the polyisocyanate resin sufficiently quickly. In particular, without the presence of filler, the development of large amounts of heat which sets in spontaneously results in the decomposition of the material. The high filler content, however, considerably limits the breadth of variation in the industrial production of the prepolymeric epoxy-resin mixture and, consequently, also of the epoxy-resin moulding compound, since any adaptation of the formulation to the processing and moulded material properties already has to be established in compounding the reaction resin mixture (of polyepoxy resin and polyisocyanate resin). In that case, however, a quick and flexible adaptation to the particular application purpose of the moulded materials is impossible.